As shown in FIG. 6 for example, a conventional automobile wheel a made of steel is typically of a two-piece construction and includes: a substantially disk-shaped wheel disk c; and a substantially cylindrical wheel rim b, in which the wheel disk c is engaged with an inner circumferential surface of the wheel rim b to be welded to the wheel rim b together into one piece (for example, Patent JP 2001-180202 A). The wheel disk c includes: a hub mounting portion f that is connected to a hub 102 of an axle 101; a hat portion h that bulges from an outer circumferential edge of the hub mounting portion f toward a surface side direction; and a disk flange d that extends from an outer circumferential edge of the hat portion h toward a back side direction. The hub mounting portion f, the hat portion h, and the disk flange d are arranged concentrically. The hat portion h includes: a hat top circumferential portion p that protrudes toward the surface side direction; and a hat inner inclined circumferential portion g that is continuously formed from the hat top circumferential portion p to the outer circumferential edge of the hub mounting portion f and that is formed circumferentially with one inner protruding portion k that protrudes toward a surface side. The hat portion h also includes a hat outer inclined circumferential portion q that is continuously formed from the hat top circumferential portion p to the disk flange d and that is formed circumferentially with one outer protruding portion m that protrudes toward the surface side into a curved shape.
The hat portion h of the wheel disk c provides desired stiffness. The hat portion h of the desired stiffness is very effective in minimizing deformation of the wheel disk c due to various loads that are imposed during car driving. Thus the automobile wheel a can have high overall strength. Particularly, under a lateral force loading on the car during driving, a bending moment acts on the wheel disk c. The bending moment is relatively large enough to bend the wheel disk c in a surface-side to back-side direction. The hat portion h provides high stiffness for the bending moment and minimizes the deformation of the wheel disk c. This allows the automobile wheel a to have high strength and to provide increased driving stability.